docs: static initialization of spinlocks is OK

Static initialization of spinlocks is preferable to dynamic initialization
when it is practical.  This patch updates documentation for consistency
with comments in spinlock_types.h.

Signed-off-by: Ed L. Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>